This invention relates to an analog-digital converter (hereinafter abbreviated as "A-D converter") and more particularly to an A-D converter which can automatically carry out zero adjustment and full-scale adjustment with high precision and attain ratiometric conversion by causing an analog input voltage and reference voltage to have the same polarity, while adapting the dual slope system.
To date, a dual slope type A-D converter has been widely accepted. FIG. 1 shows the principle and circuit arrangement of the prior art A-D converter and FIG. 2 sets forth an operating waveform. Referring to FIG. 1, reference numeral V.sub.S denotes an input voltage being measured; -V.sub.R a reference voltage; V.sub.C a comparison reference voltage; 1 a D.C. amplifier;and 2 an analog comparator. Where an input terminal 3 is impressed with the input voltage V.sub.S for a prescribed time T.sub.1 with switch means 4 set in the indicated condition, then an output voltage V.sub.O from the D.C. amplifier 1 decreases from the reference level V.sub.C, as shown in FIG. 2. The angle of inclination presented by said decrease is defined by the time constant of an integration circuit, that is, a product arrived at by multiplying the resistance R of an input resistor and the capacitance C of an integration capacitor. Next, the operation of the switch means 4 is changed over to the input terminal 5 to supply an input voltage -V.sub.R to the A-D converter. As the result, the output voltage V.sub.O rises to intersect the reference level V.sub.C in a time T.sub.2. The angle of inclination shown by the increased output voltage V.sub.O is also defined by the time constant C.sub.R of the integration circuit. The following relationship is established between the fall and rise of the output voltage V.sub.O : EQU v.sub.s t.sub.1 + ( -v.sub.r)t.sub.2 + 0
thus (V.sub.S)/V.sub.R = ( T.sub.2 /T.sub.1). Times T.sub.1, T.sub.2 are determined by counting clock pulses issued. In this case, the number of clock pulses counted during the time T.sub.2 represents the A-D converted value of the input voltage V.sub.S.
The conventional dual slope type A-D converter is based on the above-mentioned principle. However, the dual slope system of FIG. 1 is accompanied with two noticeable drawbacks described below.
The first drawback is that where a time difference occurs in the A-D conversion due to the presence of the offset voltage .DELTA.V of the D.C. amplifier 1, then the following ratio results between the levels of the input voltage V.sub.S and referential voltage -V.sub.R. EQU v.sub.s /v.sub.r = (1 - (.DELTA.v/v.sub.r)t.sub.2 /t.sub.1 - .DELTA.v/v.sub.r,
and consequently the ratio V.sub.S /V.sub.R fails to denote the ratio T.sub.2 /T.sub.1. To eliminate this drawback, various processes have been proposed, for example, the process of manually carrying out the zero adjustment and full-scale adjustment of the offset voltage of the D.C. amplifier 1 or effecting the zero and full-scale adjustments thereof by extracting the offset voltage to overlap it on the input voltage and reference voltage.
The second drawback is that the different polarities of the input voltage and reference voltage present difficulties in effecting ratiometric conversion. This ratiometric conversion can be only attained when the input voltage and reference voltage have the same polarity. With the prior art A-D converter, therefore, it was necessary for said ratiometric conversion to provide a voltage V.sub.R having the same level as the reference voltage -V.sub.R and the opposite polarity thereto. However, it was extremely difficult for forming of voltage of the positive and negative polarities to have exactly the same amplitude. Any difference in the amplitude directly led to errors in the operation of an A-D converter, making it impossible to obtain fully accurate values of ratiometric conversion.
Referring to the above-mentioned two noticeable drawbacks of the prior art A-D conversion, a number of automatic means have been proposed for the zero and full-scale adjustments of the offset of a D.C. amplifier, whereas no satisfactory method has been developed to realize accurate ratiometric conversion.